The present invention relates to a disk-array apparatus, and more particularly to a differential-LU (logical unit) management technology in a snapshot.
In a computer system, there exists a case of creating the duplication of a storage volume into which original data has been stored, such as a case of backing up data to be stored into the storage apparatus. In this case, a condition becomes necessary that, concerning the storage volume into which the original data has been stored and a storage volume into which the duplicated data is stored, their contents coincide with each other and consistency therebetween is maintained. This condition requires that an updating of the storage volume into which the original data has been stored be stopped until the creation of the duplication has been completed. In versatility systems such as the computer system, however, it is impossible in some cases to stop an access to the storage volume into which the original data has been stored.
In view of this situation, a technology called “snapshot” has been developed, which is as follows: With respect to the storage volume into which the original data has been stored and the storage volume into which the duplicated data is stored, consistency therebetween at a certain point-in-time is maintained. This is performed in order that, even if the original data has been updated after the certain point-in-time, the original data at the certain point-in-time will be able to be referred to.
According to the snapshot technology, if the original data is to be updated after the point-in-time when consistency should be maintained, the data at the point-in-time when consistency should be maintained is stored into another storage volume. Namely, if the original data has been not updated, the original data remains the same and unchanged. Otherwise, if the original data is to be updated, the data at the point-in-time when consistency should be maintained is stored into another storage volume. (Refer to, e.g., JP-A-2001-306407.)
Also, a proposal has been made concerning a method of making the snapshot available in plural generations.
According to this method, a data holding unit 1 performs a normal reading/writing. A data holding unit 2 saves a snapshot image by the data holding unit 1 at a certain point-in-time. A data holding unit 3 saves history information. Here, the history information includes updated data which accompanies a data rewriting into the data holding unit 1 after a point-in-time when the snapshot image has been saved into the data holding unit 2, information for indicating a generation of the updated data, and information for indicating an updating area of the updated data. A data-reading-source selecting unit 4, in accordance with specifications of the generation and the area of a snapshot image to be read, refers to each history information saved in the data holding unit 3, and recognizes and acquires a saving location of the snapshot image to be read, and, depending on the saving location, switches the data reading source to either the data holding unit 2 or the data holding unit 3. (Refer to, e.g., JP-A-2002-278819.)